Table Of ContentRELIEF SYSTEMS HANDBOOK
Cyril F. Parry
(IChemE Industrial Fellow)
Cyril Parry was one of the first graduates in chemical
engineering at the University of Cambridge, where he studied
under Terence Fox and two past Presidents of IChemE,
Ernest Sellers and Peter Danckwerts.
The first part of his industrial career was spent in technical
development and engineering management in the petroleum
refining and synthetic rubber industries. During the second
part he worked in process design in the engineering constructing
business, where his speciality was the design of offsites and
utilities for petroleum refineries and chemical plants.
He opted for early retirement in 1988 to devote more time to
writing and consultancy.
INSTITUTION OF CHEMICAL ENGINEERS
The information in this book is given in good
faith and belief in its accuracy, but does not
imply the acceptance of any legal liability or
responsibility whatsoever, by the Institution,
the author, or by individual members of the
Working Party, for the consequences of its use
or misuse in any particular circumstances.
All rights reserved. No part of this publication
may be reproduced, stored in a retrieval
system, or transmitted, in any form or by any
means, electronic, mechanical, photocopying,
recording or otherwise, without the prior
permission of the publisher.
Published by
Institution of Chemical Engineers,
Davis Building,
165-189 Railway Terrace,
Rugby, Warwickshire CV213HQ, UK.
0 1992 Institution of Chemical Engineers
A Registered Charity
ISBN 0 85295 471 9
Cover illustration by kind permission of Broady Valves and Elfab Hughes Ltd
Printed by Antony Rowe Limited, Eastbourne, UK
The Institution’sE ngineering Practice Committee (EPC) recognised a gap in the
published literature on relief system design, operation and maintenance. This is
in contrast to the extensive theoretical treatment available for sizing of reliefs
and relieving rates which culminated in the DIERS work. To remedy this
situation the EPC established an Industrial Fellowship whose terms of reference
were:
(1 j To identify the types and varieties of relief devices, their relative reliabilities
and their applications to specific circumstances. In particular to review the
criteria used in deciding between the different devices, especially between relief
valves and bursting discs.
(2) To study the methods used to define the total relief systems for different
applications, their installation, testing and maintenance. This includes
containment systems, common venting, flpring and blowdown, vent side
protection, access for maintenance and instrumentation.
(3) To study the criteria used for vacuum protection of vessels provided with
pressure relief.
(4) To review data on the reliability of the various types of relief systems.
(5) To review the performance of relief valves in practice, such as two-phase
flow, high temperature and liquids containing solids, with reference to case
studies.
(6) To review recommended maintenance procedures for relief devices and
users’ practices, and to summarise current codes of practice for testing and
maintenance.
(7) To review and compare current methods for sizing relief systems when
vessels are engulfed in fire.
The Fellowshipr eceived substantial support from the HSE and industry
and was directed by a Steering Committee, the members of which are listed
overleaf.
iii
Mr Cyril F. Parry, formerly Principal Process Engineer with Foster
Wheeler Energy Limited, was appointed Industrial Fellow to carry out the work
which led to the publication of this handbook.
The Institution acknowledges with gratitude the work of the
Fellowship, the Steering Committee, the HSE and the support of the sponsoring
companies.
MEMBERSHIP OF THE STEERING COMMITTEE
J.J. Clifton (Chairman) UK Atomic Energy Authority
J. Coombes Boots Co. Plc
J.S. Donald Lindsey Oil Refinery Ltd
M. Finnigan BS & B Safety Systems
A.J. Green Elfab Hughes Limited
J. Harvey British Gas Plc
M. Hodson Marston Palmer Limited
A.H. Huggett BP International Limited
P. Morley Air Products Plc
F. Owen Harcros Chemicals UK Ltd
M. Pantony Health & Safety Executive
H. Phillips Bowring Marine & Energy Insurance Brokers Ltd
N. Scilly Health & Safety Executive
A.J. Wilday ICI Plc
iv
CONTENTS
...
PREFACE 111
ACKNOWLEDGEMENTS V
1. INTRODUCTION
1.1 HOW TO USE THE HANDBOOK
1.2 LIMITATIONS
1.3 TERMINOLOGY
1.4 LEGISLATION
1.5 CODES AND STANDARDS
1.6 RELIEFS YSTEM PHIL.OSOPHY
2. RELIEF DEVICES 9
2.1 SAFETVYA LVES 9
2.2 BURSTINGD iscs 19
2.3 OTHERD EVICES 24
2.4 COMBINATIONS OF RELIEFD EVICES 30
2.5 SELEnlON OF RELIEFD EVICES 32
3. DETERMINING THE SET PRESSURE AND BURSTING
PRESSURE 39
3.1 MAXIMUOMP ERATINPGRE SSURE 39
3.2 DESIGNP RESSURE 39
3.3 ACCUMULATION 42
3.4 OVERPRESSURE 42
3.5 SETP RESSURE 43
3.6 RESEATINPGR ESSURE 43
3.7 SET PRESSURE FOR MULTI-SAFETVYAL VES 43
3.8 ADJUSTINTGH E SET PRESSURE 43
3.9 BURSTINGPR ESSURE 44
3.10 COMBINATION OF RELIEFD EVICES 45
3.1 1 CERTIFIEDDI SCHARGCEA PACITY 47
vii
4. TOTAL RELIEF SYSTEMS 49
4.1 INLET CONNECTIONS 49
4.2 VENTING DIRECTLYT O ATMOSPHERE 50
4.3 COMMOVNE NTING 53
4.4 FLAME ARRESTORS 54
4.5 CONTAINMENTA ND DISPOSASLY STEMS 56
4.6 VENTS IDEP ROTECTION 61
4.7 BLOWDOWSNY STEMS 63
4.8 GROUNDFL ARES 64
4.9 ELEVATEFDLA RES 66
4.10 CORROSIVE RELIEFS TO FLARE 66
4.11 COLD RELIEFTSO FLARE 66
4.12 KNOCK-OUTD RUMS 68
4.13 SEAL DRUMIN RARE SYSTEM 70
4.14 RELIEF DISCHARGHEE ADERS 71
5. VACUUM RELIEF 73
5.1 VACUUMR ELIEFV ALVES 74
5.2 FRESSURE~VACUUMR ELIEFV ALVES 74
5.3 VACUUMB URSTINGD iscs 74
5.4 OTHERD EVICES 75
6. THERMAL RELIEF 77
6.1 CRITERIA FOR INSTALLATION 77
6.2 SIZINGTH ERMARLE LIEFV ALVES 82
6.3 CONTAINMENTO F THERMARLE LIEF 82
7. FIRE RELIEF 84
84
7.1 CODES OF PRACTICE
7.2 DESIGN METHODS 85
89
7.3 FIRE PROTECTION
91
7.4 EFFECT OF FIREO N RELIEFD EVICES
8. RELIABILITY OF RELIEF SYSTEMS 94
95
8.1 RELIEF DEVICES
99
8.2 CONTAINMENTO R DISPOSASLY STEMS
100
8.3 CASES TUDIES
...
Vlll
9. PERFORMANCE OF RELIEF DEVICES 105
9. I HIGHP RESSURE i05
9.2 HIGHT EMPERATURE 106
9.3 Low TEMPERATURE 106
9.4 CORROSIVEEN VIRONMENT 107
9.5 FOULINSGER VICE 107
9.6 TWO-PHASFEL OW 108
9.7 LIQUIDCS ONTAININSOGL IDS 108
10. INSTALLATION OF RELIEF DEVICES 110
10.1 DUPLICATIOOFN R ELIEFD EVICES 110
10.2 VALVING OF DUPLICATERDE LIEFD EVICES 111
10.3 INSTRUMENTATION 1 I3
10.4 SAFE ACCESS 1 I4
10.5 NOISE 114
10.6 PROCESS CONDITIONS 1 I5
10.7 MECHANICADLE SIGN 1 I6
11. OPERATION AND MAINTENANCE 1 I8
11.1 COMMISSIONIRNEGL IEFD EVICES I I8
11.2 TESTING OF RELIEFD EVICES I 20
11.3 MAINTENANCE 122
11.4 REPLACEMENTS 128
11.5 MAINTENANOCFE F IREIN SULATION 128
11.6 MODIFICATIONS 129
. 11.7 RECORDS 129
11.8 RESPONSIBILITIES 129
12. CURRENT AND FUTURE DEVELOPMENT 132
12.1 INSTRUMENTPERDO TECTIVSEY STEM(SIP S) 132
12.2 EMERGENCDYE PRESSURINANGD EMPTYING 135
12.3 OTHERD EVELOPMENTS 136
12.4 OTHERW ORK 137
13. CONCLUDING REMARKS 1.40
GLOSSARY OF TERMS 141
REFERENCES 151
BIBLIOGRAPHY 153
ix
APPENDICES
1.1 THE PRESSURE SYSTEMS AND TRANSPORTAGBALSEC ONTAINERS
REGULATIONI9S8 9 i54
1.2 THE HEALTHA ND SAFETY AT WORK ACT 1974 i56
1.3 CIMAH (CONTROOLF MAJORI NDUSTRIAALC CIDENHT AZARDS)
REGULATIONS 160
1.4 CAUSES OF RELIEFS ITUATIONS 162
1.5 DETERMINATIOOFN R ELIEFR ATES 166
4. I CALCULATION OF MINIMUMDI SCHARGVEEL OCITY FOR DISPERSION 171
4.2 CALCULATION OF MACHN UMBERS 172
4.3 DISPERSIOONF ATMOSPHERIGCA SESF ROMPR ESSURE RELIEFS YSTEMS 173
4.4 CALCULATIONO F RADIATIOANT GROUNLDE VEL 174
4.5 STEAM SUPPLY FOR SMOKELESS FLAREOP ERATION 175
4.6 RECOMMENDEPDU RGGEA SV ELOCITY 176
4.7 CORRELATION OF DRAGC OEFFICIENT 177
4.8 GROUNDFL ARES 178
4.9 ELEVATEFDL ARES 180
6.1 CALCULATION OF THERMARLE LIEFR ATE 191
6.2 CLASSIFICATION OF LIQUIDFSO R THERMARLE LIEF 192
8.1 RELIABILITDYA TAF ROM NCSR MAGPIES YSTEM 193
10.1 CALCULATIONO F DISCHARGREE ACTIONF ORCE 197
12. I CHEMICRAELA CTIOHNA ZARDCS ONTENTLSIS T 198
INDEX 200
X
1. INTRODUCTION
One of the most hazardous situations that can arise in the chemical process
industry is the subjection of a closed system to a pressure higher than that for
which it was designed. This can be caused by maloperation, instrument failure,
external fire, thermal expansion, exothenn or some other reason. If the system
is not protected, the overpressurem ay lead to a catastrophic failure of the system
causing mechanical damage, loss of valuable material, emission of toxic chemi-
cals and possibly loss of life.
It is therefore a prime responsibility of the process engineer to ensure
that his or her process plant is adequately designed for the expected pressure and
temperature conditions; and that any equipment under pressure is protected by
a reliable pressure relief device that will limit the pressure rise to a permissible
level. Following installation, the device must be carefully maintained to ensure
that it continues to work effectively throughout its service life.
The pressure relief system may be considered in three separate parts:
the pressure relief device itself, its connection to the equipment which it protects,
and the disposal arrangement downstream of the relief device.
The purpose of this handbook is to advise on some of the most difficult
aspects of the relief system: the choice of relief device and the selection of the
disposal or containment system; and to discuss the reliability and maintenance
of the chosen system. The sizing of the relief device is outside the scope of this
handbook, but Section 1.6 considers the philosophy of relief with some further
guidance in Appendices 1.4 and 1.5.
The terms of reference are quoted in the Preface whilst the scope of the
handbook and its limitations are outlined in Sections 1.2 and 1.3. Section 1.4
describes the relevant British legislation and Section 1.5 deals with the applica-
tion of Codes and Standards.
1.1 HOW TO USE THE HANDBOOK
The first question is: is pressure or vacuum relief required, or can the vessel be
designed to be inherently safe? An inherently safe vessel is one which is designed
1
RELIEF SYSTEMS HANDBOOK
to withstand the highest pressure or vacuum to which it can be subjected. This
question is discussed in the introduction to Chapter 2.
If pressure relief is essential then selecting the relief device or devices
is the next step. The factors governing this selection are described in Chapter 2
with a decision tree in Figure 2.15. Chapter 5 deals with special devices for
vacuum relief. The specification of the relief device is considered in Chapter 3
with particular emphasis on the pressure regime.
The selection of the containment or disposal system is treated in
Chapter 4 with a decision tree in Figure 4.4.
As mentioned in the introduction, the sizing of the relief device is only
discussed briefly in Section 1.6. However, fire relief and thermal relief are
considered to be special cases and the sizing of devices for these contingencies
is treated in Chapters 6 and 7.
The reliability and performance of relief devices are the subjects of
Chapters 8 and 9.
Chapter 10 deals with the installation of relief devices, and Chapter 11
covers their operation and maintenance. Finally, Chapter 12 looks at alternative
methods and future developments.
The handbook is primarily a review of the best current engineering
practice and does not attempt to predict the short or long term future of relief
systems. Some alternative methods of overpressure protection are discussed in
Chapter 12 but these are not necessarily recommended as a proven replacement
for the well-established current methods.
1.2 LIMITATIONS
In order to place some reasonable boundaries on the scope of the handbook the
following topics have either been excluded or only treated briefly:
1.2.1 The design of relief systems for exothermic reaction vessels. This is
the subject of a forthcoming document by the Design Institute for Emergency
Relief Systems (DIERS) and another Institution Industrial Fellowship (see
Section 12.4).
1.2.2 Explosion relief for gas, dust and powder systems.
1.2.3 Relief devices for the transportation of hazardous materials.
1.2.4 Special devices, used in conjunction with relief devices, that limit the
2
